T cell recognition of antigen through the T cell receptor is the basis of a range of immunological phenomena. The T cells direct what is called cell-mediated immunity. This involves the destruction by cells of the immune system of foreign tissues or infected cells. A variety of T cells exist, including “helper” and “suppressor” cells, which modulate the immune response, and cytotoxic (or “killer”) cells, which can kill abnormal cells directly.
A T cell that recognizes and binds a unique antigen displayed on the surface of another cell becomes activated; it can then multiply, and if it is a cytotoxic cell, it can kill the bound cell.
HIV and Immunopathogenesis
In 1984 HIV was shown to be the etiologic agent of AIDS. Since that time the definition of AIDS has been revised a number of times with regard to what criteria should be included in the diagnosis. However, despite the fluctuation in diagnostic parameters, the simple common denominator of AIDS is the infection with HIV and subsequent development of persistent constitutional symptoms and AIDS-defining diseases such as a secondary infections, neoplasms, and neurologic disease. Harrison's Principles of Internal Medicine, 12th ed., McGraw Hill (1991).
HIV is a human retrovirus of the lentivirus group. The four recognized human retroviruses belong to two distinct groups: the human T lymphotropic (or leukemia) retroviruses, HTLV-1 and HTLV-2, and the human immunodeficiency viruses, HIV-1 and HIV-2. The former are transforming viruses whereas the latter are cytopathic viruses.
HIV-1 has been identified as the most common cause of AIDS throughout the world. Sequence homology between HIV-2 and HIV-1 is about 40% with HIV-2 being more closely related to some members of a group of simian immunodeficiency viruses (SIV). See Curran, J. et al., Science, 329:1357–1359 (1985); Weiss, R. et al., Nature, 324:572–575 (1986).
HIV has the usual retroviral genes (env, gag, and pol) as well as six extra genes involved in the replication and other biologic activities of the virus. As stated previously, the common denominator of AIDS is a profound immunosuppression, predominantly of cell-mediated immunity. This immune suppression leads to a variety of opportunistic diseases, particularly certain infections and neoplasms.
The main cause of the immune defect in AIDS has been identified as a quantitative and qualitative deficiency in the subset of thymus-derived (T) lymphocytes, the T4 population. This subset of cells is defined phenotypically by the presence of the CD4 surface molecule, which has been demonstrated to be the cellular receptor for HIV. Dalgleish et al., Nature 312:763 (1984). Although the T4 cell is the major cell type infected with HIV, essentially any human cell that expresses the CD4 molecule on its surface is capable of binding to and being infected with HIV.
Traditionally, CD4+ T cells have been assigned the role of helper/inducer, indicating their function in providing an activating signal to B cells, or inducing T lymphocytes bearing the reciprocal CD8 marker to become cytotoxic/suppressor cells. Reinherz and Schlossman, Cell 19:821–827 (1980); Goldstein et al., Immunol. Rev. 68:5–42 (1982).
HIV binds specifically and with high affinity, via a stretch of amino acids in the viral envelope (gp120), to a portion of the V1 region of the CD4 molecule located near its N-terminus. Following binding, the virus fuses with the target cell membrane and is internalized. Once internalized it uses the enzyme reverse transcriptase to transcribe its genomic RNA to DNA, which is integrated into the cellular DNA where it exists for the life of the cell as a “provirus.”
The provirus may remain latent or be activated to transcribe mRNA and genomic RNA, leading to protein synthesis, assembly, new virion formation, and budding of virus from the cell surface. Although the precise mechanism by which the virus induces cell death has not been established, it is believed that the major mechanism is massive viral budding from the cell surface, leading to disruption of the plasma membrane and resulting osmotic disequilibrium.
During the course of the infection, the host organism develops antibodies against viral proteins, including the major envelope glycoproteins gp120 and gp41. Despite this humoral immunity, the disease progresses, resulting in a lethal immunosuppression characterized by multiple opportunistic infections, parasitemia, dementia, and death. The failure of the host anti-viral antibodies to arrest the progression of the disease represents one of the most vexing and alarming aspects of the infection, and augurs poorly for vaccination efforts based upon conventional approaches.
Two factors may play a role in the efficacy of the humoral response to immunodeficiency viruses. First, like other RNA viruses (and like retroviruses in particular), the immunodeficiency viruses show a high mutation rate in response to host immune surveillance. Second, the envelope glycoproteins themselves are heavily glycosylated molecules presenting few epitopes suitable for high affinity antibody binding. The poorly antigenic target which the viral envelope presents allows the host little opportunity for restricting viral infection by specific antibody production.
Cells infected by the HIV virus express the gp120 glycoprotein on their surface. Gp120 mediates fusion events among CD4+ cells via a reaction similar to that by which the virus enters the uninfected cells, leading to the formation of short-lived multinucleated giant cells. Syncytium formation is dependent on a direct interaction of the gp120 envelope glycoprotein with the CD4 protein. Dalgleish et al., supra; Klatzman, D. et al., Nature 312:763 (1984); McDougal, J. S. et al., Science 231:382 (1986); Sodroski, J. et al., Nature 322:470 (1986); Lifson, J. D. et al., Nature 323:725 (1986); Sodroski, J. et al., Nature 321:412 (1986).
Evidence that the CD4-gp120 binding is responsible for viral infection of cells bearing the CD4 antigen includes the finding that a specific complex is formed between gp120 and CD4. McDougal et al., supra. Other investigators have shown that the cell lines, which were noninfective for HIV, were converted to infectable cell lines following transfection and expression of the human CD4 cDNA gene. Maddon et al., Cell 46:333–348 (1986).
Therapeutic programs based on soluble CD4 as a passive agent to interfere with viral adsorption and syncytium-mediated cellular transmission have been proposed and successfully demonstrated in vitro by a number of groups (Deen et al., Nature 331:82–84 (1988); Fisher et al., Nature 331:76–78 (1988); Hussey et al., Nature 331:78–81 (1988); Smith et al., Science 238:1704–1707 (1987); Traunecker et al., Nature 331:84–86 (1988)); and CD4 immunoglobulin fusion proteins with extended halflives and modest biological activity have subsequently been developed (Capon et al., Nature 337:525–531 (1989); Traunecker et al. Nature 339, 68–70 (1989); Byrn et al., Nature 344:667–670 (1990); Zettlmeissl et al., DNA Cell Biol. 9:347–353 (1990)). Although CD4 immunotoxin conjugates or fusion proteins show potent cytotoxicity for infected cells in vitro (Chaudhary et al., Nature 335:369–372 (1988); Till et al., Science 242:1166–1168 (1988)), the latency of the immunodeficiency syndrome makes it unlikely that any single-treatment therapy will be effective in eliminating viral burden, and the antigenicity of foreign fusion proteins is likely to limit their acceptability in treatments requiring repetitive dosing. Trials with monkeys affected with SIV have shown that soluble CD4, if administered to animals without marked CD4 cytopenia, can reduce SIV titer and improve in vitro measures of myeloid potential (Watanabe et al., Nature 337:267–270 (1989)). However a prompt viral reemergence was observed after treatment was discontinued, suggesting that lifelong administration might be necessary to prevent progressive immune system debilitation.
T Cell and Fc Receptors
Cell surface expression of the most abundant form of the T cell antigen receptor (TCR) requires the coexpression of at least 6 distinct polypeptide chains (Weiss et al., J. Exp. Med. 160:1284–1299 (1984); Orloffhashi et al., Nature 316:606–609 (1985); Berkhout et al., J. Biol. Chem. 263:8528–8536 (1988); Sussman et al., Cell 52:85–95 (1988)), the α/β antigen binding chains, the three polypeptides of the CD3 complex, and ζ. If any of the chains are absent, stable expression of the remaining members of the complex does not ensue. ζ is the limiting polypeptide for surface expression of the complete complex (Sussman et al., Cell 52:85–95 (1988)) and is thought to mediate at least a fraction of the cellular activation programs triggered by receptor recognition of ligand (Weissman et al., EMBO J. 8:3651–3656 (1989); Frank et al., Science 249:174–177 (1990)). A 32 kDa type I integral membrane homodimer, ζ (zeta) has a 9 residue extracellular domain with no sites for N-linked glycan addition, and a 112 residue (mouse) or 113 residue (human) intracellular domain (Weissman et al., Science 238:1018–1020 (1988); Weissman et al., Proc. Natl. Acad. Sci. USA 85:9709–9713 (1988)). An isoform of ζ called η (eta) (Baniyash et al., J. Biol. Chem. 263:9874–9878 (1988); Orloff et al., J. Biol. Chem. 264:14812–14817 (1989)), which arises from an alternate mRNA splicing pathway (Jin et al., Proc. Natl. Acad. Sci. USA 87:3319–3233 (1990)), is present in reduced amounts in cells expressing the antigen receptor. ζ-η heterodimers are thought to mediate the formation of inositol phosphates, as well as the receptor-initiated programmed cell death called apoptosis (Merćep et al., Science 242:571–574 (1988); Merćep et al., Science 246:1162–1165 (1989)).
Like ζ and η, the Fc receptor-associated γ chain is expressed in cell surface complexes with additional polypeptides, some of which mediate ligand recognition, and others of which have undefined function. γ (gamma) bears a homodimeric structure and overall organization very similar to that of ζ and is a component of both the mast cell/basophil high affinity IgE receptor, FcεRI, which consists of at least three distinct polypeptide chains (Blank et al., Nature 337:187–189 (1989); Ra et al., Nature 241:752–754 (1989)), and one of the low affinity receptors for IgG, represented in mice by FcγRIIα (Ra et al., J. Biol. Chem. 264:15323–15327 (1989)), and in humans by the CD16 subtype expression by macrophages and natural killer cells, CD16TM (CD16 transmembrane) (Lanier et al., Nature 342:803–805 (1989); Anderson et al., Proc. Natl. Acad. Sci. USA 87:2274–2278 (1990)) and with a polypeptide of unidentified function (Anderson et al., Proc. Natl. Acad. Sci. USA 87:2274–2278 (1990)). Recently it has been reported that γ is expressed by a mouse T cell line, CTL, in which it forms homodimers as well as γ-ζ and γ-η heterodimers (Orloff et al., Nature 347:189–191 (1990)).
The Fc receptors mediate phagocytosis of immune complexes, transcytosis, and antibody dependent cellular cytotoxicity (ADCC) (Ravetch and Kinet, Annu. Rev. Immunol. 9:457–492 (1991); Unkeless et al., Annu. Rev. Immunol 6:251–281 (1988); and Mellman, Curr. Opin. Immunol. 1:16–25 (1988)). Recently it has been shown that one of the murine low affinity Fc receptor isoforms, FcRγIIIB1, mediates internalization of Ig-coated targets into clathrin coated pits, and that another low affinity receptor, FcrγIIIA mediates ADCC through its association with one or more members of a small family of ‘trigger molecules’ (Miettinen et al., Cell 58:317–327 (1989); and Hunziker and Mellman, J. Cell Biol. 109:3291–3302 (1989)). These trigger molecules, T cell receptor (TCR) ζ chain, TCR η chain, and Fc receptor γ chain interact with ligand recognition domains of different immune system receptors and can autonomously initiate cellular effector programs, including cytolysis, following aggregation (Samelson et al., Cell 43:223–231 (1985); Weissman et al. Science 239:1018–1020 (1988); Jin et al., Proc. Natl. Acad. Sci. USA 87:3319–3323 (1990); Blank et al., Nature 337:187–189 (1989); Lanier et al., Nature 342:803–805 (1989); Kurosaki and Ravetch, Nature 342:805–807 (1989); Hibbs et al., Science 246:1608–1611 (1989); Anderson et al., Proc. Natl. Acad. Sci USA 87:2274–2278 (1990); and Irving and Weiss, Cell 64: 891–901 (1991)).
In drawing parallels between the murine and human low affinity Fc receptor families, however, it has become clear that the human FcRγIIA and C isoforms have no murine counterpart. In part because of this, their function has yet to be defined.
Because humoral agents based on CD4 alone may have limited utility in vivo, previous work explored the possibility of augmenting cellular immunity to HIV. Preparations of protein chimeras in which the extracellular domain of CD4 is fused to the transmembrane and/or intracellular domains of T cell receptor, IgG Fc receptor, or B cell receptor signal transducing elements have been identified (U.S. Ser. Nos. 07/847,566 and 07/665,961, hereby incorporated by reference). Cytolytic T cells expressing chimeras which include an extracellular CD4 domain show potent MHC-independent destruction of cellular targets expressing HIV envelope proteins. An extremely important and novel component of this approach has been the identification of single T cell receptor, Fc receptor, and B cell receptor chains whose aggregation suffices to initiate the cellular response. One particularly useful application of this approach has been the invention of chimeras between CD4 and ζ, η, or γ that direct cytolytic T lymphocytes to recognize and kill cells expressing HIV gp120 (U.S. Ser. Nos. 07/847,566 and 07/665,961, hereby incorporated by reference).